#include <stdio.h>
int main()
{
	int n;
	scanf("%d", &n);
	int a[31];
	a[1] = 2; a[2] = 3;
	for (int i = 3; i <= 30; i++)
	{
		a[i] = a[i - 1] + a[i - 2];
	}
	double sum = 0;
	if (n == 1)  sum = 2.0000;
	else
	{
		for (int i = 2; i <= n; i++)
		{
			double d = 2.0000 + 1.0*a[i] / a[i - 1];
			sum += d;
		}
	}
	printf("%.4f", sum);
  return 0;
}